The present invention relates generally to digital signal processing and more particularly to digital signal processing using optical noncollinear second harmonic generation.
When two noncollinear identical optical beams pass through a second harmonic generating (SHG) crystal, such as KDP, depending on the beam propagation geometry for a 90 degree phase-matching, a frequency-doubled (i.e. second-harmonic or SH) output signal NSHG can be generated. The noncollinear second harmonic generated output signal NSHG emerges with an angle that bisects the intersection angle .phi. of the two input beams. When pulsed inputs are used, the NSHG signal spatial profile represents the input temporal autocorrelation function. Since the NSHG signal can have a femtosecond response, and since the input/output frequencies are well separated and the phase matching condition acts as an angular filter, NSHG has widely been used in the past as a background free detection method for measuring temporal information down to as low as about 8 femtoseconds.
An NSHG device can be viewed as an optical Boolean logic AND gate, i.e. a SH output is generated only when both fundamental inputs are present. Using either frequency or polarization filtering, the NSHG output signal can easily be isolated.
The present invention as will hereinafter be described is based on the discovery of the application of a noncollinear second harmonic generated (NSHG) switch array element-based network to various ultrafast parallel all optical digital signal processing computations. These computations, range from binary scalar to vector multiplications, from matrix-vector to matrix-matrix multiplications, as well as from residue addition/subtraction to multiplication mapping operations, etc. In general, this NSHG-based computing network is suitable for but not limited to any application where a large number of parallel AND operations are required.
In many digital computation applications, arrays of a large number of AND gates are needed. For example, to perform either digital multiplication or digital convolution/correlation of two N-bit binary numbers, as many as N AND operations are used. For the fast multiplication, it is preferred to implement these AND functions in parallel. Because of inherent problems associated with electronic circuitry, the fastest AND gate speed is limited to the order of nanoseconds. There are technologies, using nonlinear optical etalons, to perform faster parallel AND switching. However, using any known nonlinear material, the size of the multi-reflection etalon cavity can not be made small enough to perform femtoseconds AND operation. On the other hand, a NSHG crystal has a femtosecond response so that for some AND-based special purpose computations where processing speed is the highest priority, it is an optimum device.
For performing digital optical multiplication, the existing digital multiplication via analog convolution (DMAC) method uses two processors: a convolver that performs either a time-integrating or a space-integrating binary convolution and an analog-to-digital (A/D) converter that converts the convolution result from a mixed-binary representation (MBR) to a binary output. In the present invention based on an NSHG AND gate array, two, one for time-integrating and another for space integrating ultrafast, all-optical DMAC schemes are presented.
Accordingly, it is an object of this invention to provide a new and improved technique for performing multiplication oriented type signal processing operations.